The present invention relates to an aqueous pigment dispersion, and more particularly concerns an aqueous pigment dispersion containing carbon black having improved long-term aqueous dispersing property as a colorant and a preparation method thereof as well as an aqueous dispersion capable of increasing a writing or printing density and a preparation method thereof.
Conventionally, aqueous dye ink containing a black dye has been mainly used as a recording solution used for writing tools and ink-jet printers. In recent years, in order to improve picture quality and durability of a recorded object, aqueous pigment ink using a pigment such as carbon black has received much attention.
For example, Japanese Patent Laid-Open Publication No. 6074/1989 and Japanese Patent Laid-Open Publication No. 31881/1989 have disclosed aqueous pigment ink in which carbon black is dispersed by using a surfactant and a polymer dispersant.
However, in this type of ink, when content of the colorant in ink is increased in order to raise printing density of a recorded object, the problem arises of which ink viscosity rapidly increases at the same time. Moreover, in order to stably disperse carbon black, it is necessary to add excessive amounts of a surfactant and a polymer dispersant; however, these may cause generation of bubbles and degradation in antifoaming property, resulting in degradation of printing stability, especially in the case of the application of ink-jet recording ink.
In order to solve the above-mentioned problems, Japanese Patent Laid-Open Publication No. 3498/1996 and Japanese Patent Laid-Open Publication No. 120958/1998 have disclosed an aqueous pigment dispersion in which carbon black is wet-oxidized by using hypohalite so that a group containing certain amount of active hydrogen (for example, a carboxyl group or a hydroxyl group) or a salt thereof is introduced onto a surface of carbon black, thereby allowing the surface modified carbon black itself to spontaneously disperse without the need of a surfactant and a polymer dispersant.
On the other hand, with respect to a refining method for macromolecule chromophore (MMC) for forming a pigment for ink-jet ink, Japanese Patent Laid-Open Publication No. 222573/1999 has disclosed a method in which: an aqueous solution of MMC is prepared, a liquid having desired counter ion is added to the solution, this is membrane-filtered, and the liquid addition and the filtration are repeated so that crust formation and corrugation are reduced, thereby making it possible to improve reliability of ink, such as long-term ink stability.
The first objective of the present invention is to further improve the above-mentioned surface modified carbon black in its initial dispersing property to water and in its long-term dispersing property, and consequently to provide an aqueous pigment dispersion that is less susceptible to aggregation of carbon black particles and has superior dispersion stability. The second objective of the present invention is to provide an aqueous pigment dispersion that provides superior printing density when used as ink-jet ink.
In general, surface modified carbon black, obtained by wet-oxidizing carbon black by using hypohalite, contains a great amount of alkali metal cation and halogen ion, in particular, chlorine ion. This is because upon oxidizing carbon black, hypohalite generates an alkali metal cation and halogen ion.
The inventors of the present invention have found that amounts and kinds of alkali metal cation and halogen ion give influences on the surface modified carbon black in its dispersing stability to water and in its printing density and the like when used as ink-jet ink. Then, they have further found that it is possible to greatly improve the surface modified carbon black in its dispersing stability to water by limiting concentration of alkali metal cation and halogen ion contained in the aqueous pigment dispersion, and also to greatly improve its printing density as ink jet ink by specifying kinds of the alkali metal cation; thus, the present invention has been accomplished.
The present invention provides an aqueous pigment dispersion containing surface modified carbon black that is obtained by wet-oxidizing carbon black by using hypohalous acid and/or a salt thereof, and alkali metal cation, and the aqueous pigment dispersion is characterized in that concentration of the alkali metal cation is not more than 2% by weight.
Moreover, the present invention provides an aqueous pigment dispersion containing surface modified carbon black that is obtained by wet-oxidizing carbon black by using hypohalous acid and/or a salt thereof, alkali metal cation and halogen ion, and the aqueous pigment dispersion is characterized in that concentration of the alkali metal cation is not more than 2% by weight and concentration of the halogen ion is not more than 1000 ppm.
With respect to the alkali metal cation, potassium ion and rubidium ion are preferably used.
An aqueous pigment dispersion of the present invention is preferably prepared by using a method that has the steps of: finely dispersing carbon black into water; oxidizing the carbon black by using hypohalous acid and/or a salt thereof; and adjusting concentration of alkali metal cation in the solution to not more than 2% by weight. This method may also include the step of adjusting concentration of halogen ion to not more than 1000 ppm, and may further include the step of substituting alkali metal ion in the solution by potassium ion.
An aqueous pigment dispersion of the present invention contains surface modified carbon black that is obtained by wet-oxidizing carbon black by using hypohalous acid and/or a salt thereof, and in the aqueous pigment dispersion, alkali metal cation and halogen ion generated upon wet-oxidizing carbon black are removed to not more than certain amount.
Here, the alkali metal cation refer to lithium ion (Li+), sodium ion (Na+), potassium ion (K+), rubidium ion (Rb+), cesium ion (Cs+) and the like. Concentration of alkali metal cation in the aqueous pigment dispersion may be measured by using, for example, an atomic absorption analysis method.
In this case, halogen ion refers to chlorine ion (Clxe2x88x92), bromine ion (Brxe2x88x92), iodine ion (Ixe2x88x92) and the like. Amount of halogen ion in the aqueous pigment dispersion may be measured by using, for example, an ion electrode.
With respect to carbon black as raw materials, coloring carbon black generally available in the market may be used. More specifically, when classified based upon pH, any of acidic carbon black, neutral carbon black and basic carbon black may be used, and when classified based upon a manufacturing method, any of furnace-type carbon black, channel-type carbon black, acetylene-type carbon black and thermal-type carbon black may be used.
Specific examples of carbon black used in the present invention include: #900, MA600 and MA77 (made by Mitsubishi Kagaku K.K.), Color Black FW18, Color Black S170, Special Black 4A and Printex 30 (made by Degussa Corp.), Monarch 880, Regal 250R, Regal 400R and Regal 660R (made by Cabot Corp.), Raven 1040, Raven 1255 and Conductex SC ULTRA (made by Columbian Chemicals Co.). Here, these are only examples of preferable carbon black, and the present invention is not intended to be limited by these.
In the present invention, such carbon black as raw materials is wet-oxidized in water by using hypohalous acid and/or a salt thereof. Specific examples of the hypohalous acid and/or a salt thereof include: sodium hypochlorite and lithium hypochlorite, sodium hypobromite, and lithium hypobromite; and sodium hypochlorite is particularly preferable from the viewpoint of reactivity and costs.
The oxidizing reaction is carried out as follows: Carbon black and hypohalite (for example, sodium hypochlorite) having an effective halogen concentration of 5 to 15% by weight are loaded into an appropriate amount of water, and this is stirred for not less than 5 hours, preferably, approximately 10 to 15 hours, at not less than 0xc2x0 C., preferably at room temperature to 100xc2x0 C. At this time, the carbon black is preferably ground beforehand, or is oxidized while being simultaneously ground. With respect to the grinding method, this is put in a ball mill, an attriter, a colloid mill or a sand mill together with beads of glass, zirconia, alumina, stainless magnetism and the like, and ground therein. In the case of carbon black that is easily ground, it is ground in a rotary homogenizer or a supersonic homogenizer.
Amount to be used of the hypohalite is different depending on its kind and effective halogen concentration; however, in general, it is from 0.5 to 150% by weight, more preferably, 1 to 75% by weight, at 100% conversion based upon weight of the carbon black.
The resulting surface modified carbon black has an oxygen content of not less than approximately 3% by weight, more preferably, not less than approximately 5% by weight. As a result of oxidation by a method of the present invention, oxygen content of carbon black increases to approximately several times larger than oxygen content of the carbon black before the treatment.
Oxygen content of the surface modified carbon black is measured by the xe2x80x9cinert gas-infrared absorption methodxe2x80x9d. In this method, the surface modified carbon black is heated in a flow of inert gas such as helium and the like, oxygen is extracted as carbon monoxide, and amount thereof is measured by the infrared absorption method. This method is described in JIS Z 2613-1976 in detail.
In general, in a reaction between carbon black and hypohalite, oxygen-based functional groups, such as a carboxyl group, a hydroxyl group, a carbonyl group and a lactone group, are generated on a surface of carbon black. The greater the above-mentioned oxygen content, the greater amount of the oxygen-based functional groups.
The surface modifying carbon black used in an aqueous pigment dispersion of the present invention is preferably have high oxygen content. The surface modifying carbon black of this type exhibits particularly superior water dispersing property.
Next, a dispersion of the surface modified carbon black after having been oxidized is filtered (in hot state), and the resulting wet cake is again dispersed in water; thereafter, beads and coarse particles are removed from this by using a metal mesh.
Next, by-product salt containing alkali metal cation that are generated upon wet-oxidizing carbon black, and halogen ion, are further removed from the surface modified carbon black obtained through oxidization. The alkali metal cation and the halogen ion are removed by washing the surface modified carbon black with water. The washing is preferably carried out by using deionized water such as ion exchanged water.
In general, washing with water is carried out by allowing deionized water to pass through the surface modified carbon black with a separation film that does not transmit the surface modified carbon black being placed. With respect to the separation film, a reverse permeation film or a ultrafiltration film is used. With respect to the reverse permeation film, NTR-7250, NTR-719HF and NTR-7410 made by Nitto Denko K.K. may be used, and with respect to the ultrafiltration film, NTU-2120, NTU-3150 and NTU-3520 made by Nitto Denko K.K. may be used.
The washing with water of the surface modified carbon black is preferably carried out until concentration of alkali metal cation has been reduced to not more than 2% by weight, preferably, not more than 1% by weight, more preferably, not more than 0.8% by weight. If concentration of alkali metal cation is not less than 2% by weight, aggregation of pigment particles becomes easily to occur, initial dispersing property and long-term dispersing property of pigment to water is not properly maintained; consequently, it becomes difficult to achieve the advantage of the present invention.
The washing with water of the surface modified carbon black is preferably carried out until concentration of halogen ion has been reduced to not more than 1000 ppm, preferably, not more than 500 ppm, more preferably, not more than 300 ppm. If concentration of halogen ion is not less than 1000 ppm, aggregation of pigment particles becomes easily to occur, initial dispersing property and long-term dispersing property of pigment to water is not properly maintained; consequently, it becomes difficult to achieve the advantage of the present invention.
Concentration of alkali metal cation may be accurately determined by using, for example, the atomic absorption analysis method. Alternatively, conductivity of the dispersion at the time of washing, or eluent water after washing is measured by a conductivity meter, thereby approximate concentration may indirectly be determined. By using such an indirect method, it becomes possible to easily know approximate concentration of alkali metal cation by simply dipping an electrode into the dispersion at the time of washing or into eluent water after washing, and also to know approximate concentration of alkali metal cation at real time without suspending the preparation process.
For example, in the present embodiment, it has been found through experiences that, in the case when conductivity of an aqueous solution is not more than 10 mS/cm, concentration of alkali metal cation is not more than 2% by weight, and that in the case when conductivity of an aqueous solution is not more than 5 mS/cm, concentration of alkali metal cation is not more than 1% by weight.
It is preferable that concentration of halogen ion is measured by using an ion electrode. This is because concentration of halogen ion is easily measured by simply dipping an electrode into the dispersion at the time of washing or into eluent water after washing. By using an ion electrode, it becomes possible to easily know concentration of halogen ion accurately at real time without suspending the preparation process.
With respect to a preferable ion electrode, for example, Chloride Ion Electrode 8002-10C, Bromide Ion Electrode 8005-10C, Iodide Ion Electrode 8004-10C and the like made by Horiba Seisakusho K.K. are listed.
Thereafter, pH of a dispersion of the surface modified carbon black may be adjusted, if necessary. With respect to a pH adjusting agent, hydroxide of alkali metal, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, aqueous ammonia, triethanol amine, diethanol amine and lower alkyl amine, or amines are listed. Preferably, potassium hydroxide and rubidium hydroxide are used.
Lithium hydroxide, sodium hydroxide or potassium hydroxide may be used as a pH adjusting agent; however, each of these has to be added so that amount of alkali metal cation contained in the aqueous pigment dispersion is not more than 2% by weight. If amount of the alkali metal cation is not less than 2% by weight, pigment particles easily aggregate, initial dispersing property and long-term dispersing property of pigment to water is not properly maintained.
Moreover, an antiseptic agent and a mildewproofing agent may be added to a dispersion of the present invention.
Furthermore, alkali metal cation (for example, sodium ion and lithium ion) in an aqueous pigment dispersion containing the surface modified carbon black may be substituted by potassium ion and rubidium ion. This is because an aqueous pigment dispersion of the present invention containing potassium ion and rubidium ion as alkali metal cation exhibits superior printing density when used as ink-jet ink.
It has been unknown the reason why the surface modified carbon black having potassium salt and rubidium salt exhibits printing density (OD value) higher than that having sodium salt and lithium salt; however, there is a tendency that those having greater ion radius have higher OD value (see Examples 4 to 8).
With respect to a method for substituting alkali metal ion in the aqueous pigment dispersion by potassium ion, the oxidized pigment liquid is filtered to obtain surface modified carbon black, and this is washed, and re-dispersed in water (preferably, in ion exchanged water). To this is added mineral acid, preferably hydrochloric acid, to adjust pH to acidic, preferably, to not more than 2.5, and this is stirred for approximately two hours, and filtered to remove salt generated by a neutralization process. Then, the resulting wet cake of the surface modified carbon black is again dispersed in ion exchanged water, and to this is added, for example, potassium hydroxide or rubidium hydroxide, or its aqueous solution, serving as a desired alkali metal ion adding agent, to adjust pH thereof from neutral to alkali, and this is held for approximately four hours while being stirred, if necessary; thus, the target aqueous pigment dispersion is obtained.
Thereafter, a ultrafiltration film or a reverse permeation film and the like is used to adjust concentration of alkali metal cation to not more than 2% by weight, preferably, not more than 1% by weight, more preferably, not more than 0.5% by weight; thus, an aqueous pigment dispersion of the present invention is obtained.
In general, the surface modified carbon black of the present invention is contained in the range of 1 to 40% by weight, preferably, in the range of 5 to 20% by weight, based on the whole amount of an aqueous pigment dispersion. The content of the surface modified carbon black less than 1% by weight causes an insufficient coloring density, and the content exceeding 30% by weight makes the surface modified carbon black more susceptible to aggregation, resulting in high viscosity during long-term storage or generation of precipitation; consequently, these contents are not preferable.
Average particle size of the surface modified carbon black in an aqueous pigment dispersion of the present invention is preferably not more than 300 nm, more preferably, not more than 150 nm. The average particle size of the surface modified carbon black exceeding 300 nm tends to cause precipitation of pigment.